San Francisco's escalating yearly tab for pension costs for city workers could hit nearly $700 million in 2014 - an amount larger than the current annual operating budget for San Francisco General Hospital.

This year, the city contribution is less than $300 million, officials said.

City voters next month will be asked to change pension rules by shifting more of the funding burden onto city workers and away from the government treasury to save taxpayers between $300 million and $500 million cumulatively over the next 25 years, according to an analysis by the city controller.

Even by the controller's calculations, the numbers are a moving target based on estimates that consider such factors as wage fluctuation, investment returns, labor agreements and the number of city employees.

What is more certain is that even if Proposition D were approved, it would do nothing to address the city's immediate financial crunch, dominated by a projected $483 million deficit that needs to be closed for the new fiscal year that starts July 1.

But over time, the savings would kick in and ease the strain on the city budget, Prop. D backers assert.

"It's not going to fix the problem, but it's a start," said Supervisor Sean Elsbernd. He, with Mayor Gavin Newsom, was an early promoter of the plan. It was tweaked by the Board of Supervisors to address labor concerns before it landed on the ballot.

"I don't think there's a more important issue that we have to own up to, and that's the pension." Newsom said. "The costs are something that's crowding out our discretionary budget."

The ballot measure has no formal opposition - even among public employee unions - and has backing from a wide range of influential players, from the Bay Guardian weekly newspaper on the left to the more conservative San Francisco Chamber of Commerce.

The proposed pension changes would apply only to new city employees hired after July 1 of this year; city workers already on the payroll would not be affected. By 2032, most city employees would be covered by the new rules.

The proposed measure would:

-- Change the way pensions are calculated. Currently, pensions are based on the pay earned during the last year on the job, when compensation has been known to spike. Under the proposal, they would be based on the average pay during the final two years of employment.

Elsbernd and the mayor had pushed for a three-year average, but couldn't secure majority support at the Board of Supervisors amid union pushback. The two-year compromise quelled opposition.

-- Increase the employee pension contribution to 9 percent - up from the current 7.5 percent - for public safety employees and other workers on the San Francisco payroll covered by CalPERS, the state retirement system. All other city employees who contribute to their pension fund would continue to pay the 7.5 percent rate.

-- In years when the retirement system's return on investments does better than planned - reduces contributions needed from the city's general fund - the savings would be stashed in a new dedicated fund to help cover the mounting cost of retiree health and pension benefits in future years. The city now carries an estimated unfunded liability of approximately $4 billion for the future costs of retiree health benefits alone earned by employees already on the payroll.

Prop. D is an attempt to build on a measure passed in 2008 in which voters doubled to 10 the number of years that newly hired city employees would have to be on the payroll to qualify for health benefits upon retirement.

Public Defender Jeff Adachi, meanwhile, is pushing another pension reform measure for the November ballot that would call on current employees to pick up a bigger share of the tab for their retirement benefits.

San Francisco measures

Voters will be asked to decide the following:

Measure A: Renew for another 20 years a school facilities tax to fund capital projects in San Francisco public schools. Property owners would pay $32.20 a year for single-family homes and nonresidential parcels, or $16.10 per unit for multifamily residential and mixed-use properties. The parcel tax is expected to generate $7 million a year and has been on the books since 1990.

Measure B: Authorize a $412 million earthquake bond that would shore up neighborhood fire stations and an auxiliary water-supply system used by the Fire Department.

Measure C: Change the city charter to take away the mayor's authority to appoint all 11 members of the Film Commission and limit the mayor's picks to six appointees. The remaining five members would be appointed by the Board of Supervisors.

Measure E: Require the Police Department to report how much it spends annually to protect the mayor, members of the Board of Supervisors and other dignitaries.

Measure F: Allow tenants to defer rent increases if they've seen their wages cut or experienced other financial hardship.

Measure G: Support the northern terminus for the state's planned high-speed rail route in the new Transbay Center at First and Mission streets.

Early voting runs from now until June 7 at the Elections Department at City Hall. The last day to request mail-in ballots is June 1.